Which encoder should I use for this application? This is a popular question that depends on two key factors: what the application is, and how you want to accomplish it. PLC’s can accept many types of encoder signals, from quadrature pulse train, analog, grey code, SSI, CANbus, IO-link, Ethernet/IP inclusive and Ethernet/IP for CIP Motion applications. The discussion about all these options could go on for days, so for this article let’s focus on the most common comparison: Ethernet/IP encoder vs CIP Motion Encoder.
When posed with the challenge of selecting an encoder for a specific application, it is important to understand the features and functions of each encoder type. These features and benefits can, and will, save you time and headaches as you commission your machine. Making sure you select the correct choice ahead of time is crucial because an initial misstep can lead to a mechanical nightmare later on if it needs to be switched!
Both the 842E-M/S & 842E-CM-M/S Encoders are Ultra High-resolution Absolute Encoders with an Ethernet/IP Interface. They are both available in Single & Multi Turn versions. They are used with Control Logix and Compact Logix Controllers and RSLogix/Studio 5000 Software. The 842E-M/S Encoder Resolution is 262,144 Pulses per Revolution x 1 Turn. The 842E-CM-M/S Encoder Resolution is 262,144 Pulses per Revolution x 4096 Turns. So when should you use which?
If you answered yes to the first question you want an ethernet encoder — the 842E-M/S. The 842E is a dual port ethernet encoder intended for use in a basic Ethernet system with more of a process focus. It is NOT for use in motion systems, as it is considered an IO device to Logix. The 842E Encoder can be connected in any of three network topologies: Star, Linear & Device Level Ring (DLR). An Ethernet/IP address can be set in two ways: the last octet by using the switches on the Encoder for (192.168.1.x) or by using BootP/DHCP. The 842E Encoder is configured in RSLogix / Studio 5000 Version 18 or greater.
If you want to use the encoder in a motion system you should select a CIP Motion Encoder. The 842E-CM is a dual port ethernet encoder used in CIP Sync & CIP Motion Systems with Kinetix Motion Drives. CIP Sync provides a mechanism to synchronize clocks between controllers, I/O devices and other products. The 842E-CM Integrated Motion Encoder can also be connected in any of the three topologies. An Ethernet I/P address is also set through the encoder switches or BootP/DHCP. The 842E-CM Encoder is configured in RSLogix / Studio 5000, configured into the motion group, and then considered as an AXIS to Logix. This means you can CAM, gear, or use any other commonly used motion instruction on the device. Minimum Coarse Update (CU) Rate is 4ms (controller interpolates between each CU), which is fast enough to provide performance equal to or better than a hardwired encoder. It is intended to be used with the Motion Capable Control/Compact Logix –“ERM” platform. It is not intended for use with MicroLogix, Micro800, or SLC.
Still need help deciding? Feel free to contact me and we can find the right encoder for your specific application. Rumsey also has demos available upon request.